Different types of prior art rotary polishers are used in chemical-mechanical polishing of semiconductor wafers, where a thin layer of semiconductor material is planarized during the overall semiconductor manufacturing operation. The chemical-mechanical polishers typically have a heavy, circular polishing table made from a metal material or rigid ceramic material. The top surface is usually machined flat or formed smooth, and a polishing pad is glued onto the top surface. The polishing table is a predetermined diameter that is larger than the diameter of any semiconductor wafer that will be planarized in the chemical-mechanical polishing step. An abrasive slurry is fed onto the top surface of the polishing table. Typically, a semiconductor wafer carrier is positioned over the polishing table at a position for engaging the top polishing surface. The semiconductor wafer carrier holds the semiconductor wafer through an appropriate mechanism and places the semiconductor wafer against a polishing pad that is positioned on the top surface of the polishing table.
Standard prior art polishing pads in some rotary polishers usually consist of a stack with a hard layer on top of a soft under-pad. The hard pad produces a local planarization of topographical features, while the soft layer allows the stack to conform globally to the wafer shape. The use of a soft under-layer degrades the planarization ability of the top pad. However, a single layer hard pad cannot be used on a standard rotary polisher because high spots in the wafer will polish preferentially, resulting in very poor within-wafer uniformity. In order to improve global uniformity, the downward biasing force can be increased to a very high value, about 10 pounds per square inch, which flattens the semiconductor wafer and allows the pad to conform better to the semiconductor wafer. However, within-die uniformity is degraded as the downward directed force increases.
One recently developed chemical-mechanical polishing tool uses a "belt sander" approach. A single hard pad is mounted on a thin metal belt. The linear motion of the belt is used to polish the wafer. Good global uniformity can be achieved by tailoring the pressure behind the metal belt under the wafer. However, it is not always desirable to use a reciprocating movement because the technology of most chemical-mechanical polishing tools are directed to the rotary type of polishing table. An example of a reciprocating polishing table is the rectangular configured table shown and disclosed in U.S. Pat. No. 5,908,530 to Hoshizaki et al.